1. Field of the Invention
The present invention relates to a nonvolatile semiconductor memory device which has a floating gate and upon which data can be electrically written and erased. More particularly, it relates to a nonvolatile semiconductor memory device with a structure where error-writing of data is less prone to occur.
2. Description of Related Art
Included in nonvolatile semiconductor memory devices is a so-called flash memory upon which data can be electrically written and erased.
In this flash memory, by applying a positive or negative high voltage between a source or drain region and a control gate, electrons are extracted from a floating gate so that a data is written and electrons are injected into the floating gate so that the data is erased.
Among such electrically writable and erasable flash memories is one called an ACT (Asymmetrical Contactless Transistor) type flash memory which utilizes an FN tunnel phenomenon for both writing and erasing and which has a memory-array structure where a virtual-ground system is employed.
Now, a brief explanation will be given to the virtual ground system. In the virtual ground system with such a memory-array structure that memory cells are arranged in a matrix with X and Y directions crossing substantially at a right angle (the word “substantially” is used because the angle is not necessarily required to be a right angle as long as the X and Y directions cross each other so as to form a matrix) and that wordlines extend in the X direction and bit lines extend in the Y direction, each bit line is electrically connected to a single impurity diffusion layer that constitutes both a source region of one memory cell and a drain region of an adjacent memory cell in the X direction. Therefore, in this virtual-ground system, each bit line serves both as a source wiring for one memory cell and as a drain wiring for the other memory cell, and is not fixed either as the source wiring or the drain wiring, but the source wiring (being commonly grounded) and the drain wiring are appropriately switched, as disclosed in, for example, Japanese Unexamined Patent Publication No. Hei 9(1997)-92739.
FIG. 11 is a plan view illustrating an ACT-type flash memory cell array of prior art in which the virtual-ground system is employed. FIG. 12 is a cross section taken along line X-X′ (along a wordline) of FIG. 11. FIG. 13 is a cross section taken along line Y1-Y1′ (along a bit line) of FIG. 11. FIG. 14 is a diagram illustrating an equivalent circuit of the ACT-type flash memory cell array.
Referring to FIGS. 11 to 14, C (C11, C12, . . . , Cij, . . . , C532) denotes a memory cell, WL (WL1, WL2, . . . , WL32) a word line, MBL (MBL1, MBL2, . . . , MBL6) a main-bit line, SBL (SBL1, SBL2, . . . , SBL6) a sub-bit line made of a buried impurity diffusion layer, SG (SG1 to SG4) a selection gate constituted of a switching element such as a transistor or the like, and BC a bit-line contact for electrically connecting the main-bit line MBL to the sub-bit line SBL.
In FIGS. 12 and 13, reference numeral 1 denotes a semiconductor substrate, 2 a source region made of a low-concentration n−-type impurity diffusion layer, 3 a drain region made of a high-concentration n+-type impurity diffusion layer, 4 a tunnel oxide film, 5 a floating gate, 6 a buried oxide film, 7 an ONO film (oxide film/nitride film/oxide film), 8 an interlayer dielectric film for insulating and separating the word line WL from the main-bit line MBL, 10 an isolation region, 11 a bit-line contact (corresponding to the BC of FIG. 11), and 16 a control gate (constituted of a portion of the wordline facing the floating gate 5). Reference numeral 9 denotes a capacity existing between the adjacent main-bit lines that is schematically illustrated in FIG. 12.
As shown in FIG. 12, the control gates 16 of the cells arranged in the X direction are continuously connected together to form the wordline WL (in this figure, WL3).
A impurity diffusion layer that constitutes the source region 2 of one memory cell and the drain region 3 of an adjacent memory cell in the X direction serves as the sub-bit line SBL (SBL1, SBL2, . . . ) to electrically connect the memory cells with one another along the Y direction. The sub-bit line SBL is connected to the main-bit line MBL via the selection gate SG by the bit-line contact 11 (corresponding to the BC of FIG. 11). The main-bit lines MBL are formed above the wordlines WL via the interlayer dielectric film 8.
As described above, the ACT-type memory cell has an n-channel-transistor structure provided with the lightly doped source region 2, the heavily doped drain region 3 and the floating gate 5. The asymmetric distribution of impurity concentration in the source region 2 and drain region 3 makes it possible to utilize the FN tunnel phenomenon for both writing and erasing and to realize the simple virtual-ground structure.
In other words, the memory cell array, as shown in FIG. 11 and FIG. 14 (equivalent circuit), is allowed to have the virtual-ground structure in which the sub-bit line SBL of a single impurity diffusion layer is shared by the source region 2 of one memory cell and the drain region 3 of an adjacent memory cell in the X direction. The sub-bit lines SBL are arranged in parallel to the main-bit lines MBL, while the wordlines WL are arranged to cross substantially at a right angle both the sub-bit lines SBL and the main-bit lines MBL.
In this memory cell array where the sub-bit lines arranged in the Y direction are divided by an isolation region 10 formed by injecting boron ions, utilizing p-n junctions, no field oxide film is required, making it possible to realize high integration of memory cells.
This kind of memory where the memory cells are arranged in parallel is classified as a NOR-type flash memory.
The operation principle of the ACT-type memory cell will now be explained with reference to FIGS. 14 and 15.
In a writing operation, electrons are extracted from the floating gate 5 into the drain region 3 through the FN tunnel phenomenon to lower a threshold voltage. For example, for writing the memory cell C23 (a cell to be written is referred as a selected cell), −12V is applied to the selected wordline WL3 and +4V is applied to the main-bit line MBL2, so that +4V is applied to the sub-bit line SBL2 via the selection gate SG3 from the main-bit line MBL2. As a result, +4V is applied to the drain region 3, thereby lowering the threshold voltage to 1V-2V.
At this time, the same writing voltage is applied also to the adjacent cell C13 which is not to be written (a cell which is not to be written is referred to as a non-selected cell). However, an electric field intensity at a portion of the tunnel oxide film 4 closer to the n−-type source region 2 is smaller than one at a portion of the tunnel oxide film 4 closer to the n+-type drain region 3, because a depletion layer directly below the tunnel oxide film 4 in the n−-type source region 2 serves to reduce the electric field intensity at the portion closer to the n−-type source region 2. As a result, a non-selected cell adjacent to a cell written along the line constituted of the control gates (i.e., the wordline WL3) will not be written even if the writing voltage is applied thereto. This is why the writing operation utilizing the FN tunnel phenomenon can be performed while realizing the virtual-ground structure.
In an erasing operation, +12V is applied to a selected wordline and −8V is applied to the silicon substrate 1, to the sub-bit line SBL and to the main-bit line MBL, respectively, so that electrons are injected into the floating gate 5 from the silicon substrate 1 through the FN tunnel phenomenon in a channel region to raise the threshold voltage to 4V or more. The erasing operation is can be performed per block defined as an area between the selection gate SG and the isolation region 10 or per wordline.
In a reading operation for, for example, reading the selected cell C33, +3V is applied to the selected wordline WL3, a source voltage +1V is applied to the SBL4 and a drain voltage 0V is applied to the SBL3, and it is decided whether or not the selected cell C33 is in a written state or an erased state depending on whether or not a current flows through the selected cell C33.
In a flash memory, a high voltage is applied between a drain region and a control gate such that the voltage of the control gate is negative with respect to that of the drain region for the writing and a high voltage is applied between a source region and the control gate such that the voltage of the control gate is positive with respect to that of the source region for the injection. For this purpose, a high voltage is applied to a selected main-bit line MBL for applying the high voltage to the source and drain region of a selected cell, while a non-selected main-bit line MBL connected to a non-selected cell is put in a 0V floating state.
In the aforementioned ACT-type memory, +4V is applied to the selected bit line MBL, while the non-selected bit line MBL is put in a 0V floating state.
As shown in FIG. 14, in the case of the non-selected C33 between the two selected cells C23 and C43, +4V is applied to the selected bit lines MBL2 and MBL4, respectively, while the non-selected bit line MBL3 is put in a 0V floating state.
Here, the capacities 9 exists between the adjacent main-bit lines MBL, as shown in FIG. 12. This is because the main-bit lines MBL, which are made of a conductive material such as a metal, are arranged side by side with small gaps therebetween in the interlayer dielectric layer 8, so that capacitors are formed between the adjacent main-bit lines MBL, thereby electrical coupling the adjacent main-bit lines MBL. Each capacity 9 increase as the distance between the adjacent main-bit lines MBL reduces with the advancement of the integration of the ACT-type memory.
The electrical coupling between the adjacent main-bit lines may possibly give rise to the following problem. When a main-bit line MBL is selected and a high voltage is applied thereto, an adjacent non-selected main-bit line MBL may be influenced by the high voltage and caused to float, so that electrons may be extracted in error from the floating gate 5 of a non-selected cell which is connected to the non-selected main-bit line MBL. For example, assume that the main-bit lines MBL2 and MBL4 are selected, as shown in the equivalent circuit of FIG. 14. Then, the non-selected bit line MBL3, which is capacity-coupled to the main-bit lines MBL2 and MBL4, is influenced by a high voltage to the main-bit lines MBL2 and MBL4, and the voltage of the non-selected bit line MBL3 is raised up to, for example, about 1.5 V, so that electrons are extracted from the floating gate 5 of the non-selected cell C33 into the sub-bit line SBL3. Thus, a threshold voltage of the non-selected cell C33 is lowered, resulting in occurrence of error-writing.
FIG. 17 is a distribution graph of threshold value plotted against the number of memory cells after writing of the ACT-type memory (a memory where main-bit lines are arranged on the same plane) shown in FIG. 12. This graph is for explaining error-writing by an example and corresponds to FIG. 10. Owing to the floating phenomenon of a non-selected main-bit line MBL at the time of writing, electrons are extracted from the floating gate 5 of a non-selected cell which is in an erase-state, lowering a threshold voltage of the non-selected cell. As a result, the non-selected cell is written in error. The area X is the written cells in error.
Such error-writing is performed not only in the ACT-type memory, but also in a flash memory where electrical coupling occurs between the adjacent main-bit lines so that floating of a non-selected main-bit line adjacent to a selected main-bit line is caused at the time of writing, and in a nonvolatile semiconductor memory device.
A measure to inhibit the floating of a non-selected main-bit line may be to put the non-selected main-bit line in a 0V-forced-application state (a state created by forcedly applying to it a voltage 0V externally so that it is fixed at 0V), instead of putting the non-selected main-bit line in a 0V-floating state (a state created by first setting it at 0V and next isolating it from external circuits so that it is in a floating state).
However, depending on the kind and use of a nonvolatile semiconductor memory device, it may be difficult to use the device in a 0V-forced state.
Especially in the Act-type memory, where the virtual-ground structure is used, the presence of an off-state-leakage current between a source region and a drain region of a non-selected cell produce a current flow between a selected man bit line (+4V) and a non-selected main-bit line (0V) as shown in FIG. 16, which increase current consumption at the time of writing. Also, the presence of the off-state-leakage current causes current decrease at a main-bit line resistor and a sub-bit line resistor, so that a voltage supplied to the drain region of a selected memory cell becomes lower than a voltage applied, which reduces a writing rate.